Plan to set out the butter and cream cheese about 2 hours before you plan to make the frosting if you can. That will give it plenty of time to soften up and be easy to blend.
Add the cream cheese to a large bowl and beat with a hand-held electric mixture for about a minute on high speed.
Add the butter, powdered sugar, cocoa powder, vanilla, and salt, and beat on low to start, then work up to high speed and beat until thoroughly combined and creamy, about 1 minute.
Use on cupcakes or a cake. Or store, in an airtight container or in a bowl tightly covered with plastic wrap and placed into the fridge, for up to 3 days.
Be sure that the butter and cream cheese have time to soften at room temperature before you start blending. I usually set them out 2 hours before I plan to start if possible.
Start with 1/2 cup powdered sugar and add additional to reach a sweetness you like. (I use 1 cup.)
Add 1 tablespoon milk or cream at a time at the end of blending to reach your desired consistency.
Make ahead and store in an airtight container for up to 3 days in the fridge.